1. Field of the Invention
This invention relates to a high-frequency induction heating coil for thermally fusing a raw material crystalline rod and more particularly to a high-frequency induction heating coil to be used for the growth of a semiconductor single crystal by the floating zone (FZ) method.
2. Description of the Prior Art
As a means for growing a semiconductor single crystal by an FZ method, the method which, as shown in FIG. 3, implements growth of a single crystalline rod 2 by setting fast a raw material polycrystalline rod 1 on the upper shaft and a seed of a single crystal of a small diameter on the lower shaft located directly below the raw material polycrystalline rod 1, encircling the raw material polycrystalline rod 1 with a high-frequency inducting heating coil 3, melting the raw material polycrystalline rod 1 and causing the seed crystal to immerse in the melt, and then reducing the diameter of the seed crystal thereby eliminating dislocation and meanwhile relatively rotating the raw material polycrystalline rod 1 to a heating coil 3 and moving the rod 1 in the axial direction has been well known heretofore. This growth method requires the raw material polycrystalline rod 1 to be quickly melted to the core in the narrowed molten zone. Meanwhile, for the purpose of enabling the single crystal 2 to grow stably after the zone melting without impairing uniform distribution of impurities, it is necessary that the front end of the solidified single crystal adjoining a molten zone 4 is caused to radiate heat slowly. To satisfy these requirements, a flat induction heating coil 3 as a pancake has been heretofore practically employed.
In the flat induction heating coils 3, those constructed as shown in FIG. 4 have been popularly recognized (as disclosed in JP-B-51-24,964, for example; hereinafter referred to as "first conventional technique"). In the heating coil 3 of this first conventional technique, an annularly shaped coil thereof is so formed that the cross section thereof gradually decreases in thickness toward the inner circumferential surface 7 side and opposed faces 5a, 5b on the opposite end sides of the coil 3 provided with power source terminals 6a, 6b on an outer circumferential surface 8 are close each other across a space 5 to the fullest possible extent. Owing to this construction, the coil 3 assumes symmetry of the current circuit thereof in the circumferential direction and acquires a practically uniform magnetic field distribution.
According to the heating coil 3 of the conventional technique shown in FIG. 4, since the space 5 of the heating coil 3 is formed along the faces perpendicular to the circumferential direction of the heating coil 3, an ununiform magnetic field is inevitably generated in the part in which the faces 5a, 5b are opposed to each other across the space no matter how small the space may be. Further, since electric currents flow in mutually opposite directions along the radial direction near the opposed surfaces 5a, 5b, the electromagnetic field in the vertical direction which affects the growth of crystal most seriously is doubled by the electric currents in the opposite directions and the ununiform magnetic field is all the more amplified.
When the raw material polycrystalline rod 1 and the heating coil 3 are rotated and moved relatively to each other in the presence of the ununiform magnetic field, layers containing impurities alternately in a high concentration and in a low concentration are repeatedly formed in each growth cycle per rotation owing to a local temperature difference caused by the ununiform magnetic field (hereinafter referred to as "rotational striation"). When a device is produced by the use of a single crystal containing such rotational striations, the microscopic variation of resistance in the rotational striation can cause property deviation in the product.
To eliminate this defect of the first conventional technique, a high-frequency induction heating coil 10 which, as shown in FIG. 5, has a plurality of slits 13a through 13d and 14a through 14e extended in the radial direction from the inner circumferential surface 17 side or from the outer circumferential surface 18 to halfway along the coil width (hereinafter referred to collectively as "slits 13, 14") throughout the entire thickness of the coil in the axial direction has been invented (JP-A-52-30,705, hereinafter referred to as the "second conventional technique"). In the heating coil 10 of the second conventional technique, the plurality of slits 13, 14 having the same width as a space 12 are so staggered and spaced circumferentially as to assume geometric periodicity. Consequently, the high-frequency electric current which flows on the surface of the heating coil 10 mentioned above is controlled symmetrically relative to the axis of the coil.
For the purpose of cooling the heating coil 10 of the second conventional technique constructed as shown in FIG. 5, however, it is necessary that the heating coil 10 is provided therein with flow paths capable of supplying cooling water between the inner circumferential surface 17 and the slits 14 or between the outer circumferential surface 18 and the slits 13. Thus, gaps are to be formed between the inner circumferential surface 17 and the slits 14 or between the outer circumferential surface 18 and the slits 13. When the high-frequency electric current flows along the slits 13 and 14, it takes the shortest route deviated inward from the ideal route by using the gaps between the circumferential surfaces and the slits. The heating capacity of the coil near the inner circumferential surface 17, therefore, is decreased in proportion to the size of the deviation. As a result, the convective stirring force in the central part of the molten zone 4 is weakened and the resistivity near the axis of the semiconductor single crystal 2 in process of growth is inevitably lowered.
To adjust the heat distributing property of the heating coil 10, the slits 13 and 14 must be varied in length and width. For the sake of this variation, the heating coil 10 must be elaborately remade. Thus, the adjustment of the heat distributing property cannot be readily carried out. Further, since the route for the electric current is long, the space 12 possibly discharges electricity near the power source terminals 15 and 16, so that the heating operation cannot be stably performed.